Method of coating a metal substrate with molybdenum



Feb. 26, 1957 M, HIL 2,783,164

METHOD OF COATING A METAL SUBSTRATE WITH MOLYBDENUM Filed Sept v 17, 1953 IN VEN TOR.

v Mai-Sc H ATTORNEY United States Patent i METHOD OF COATING A METALSUBSTRATE WITH MOLYBDENUM Morse Hill, Nahant, Mass., assiguor to National Research Corporation, Cambridge, "Mass, a corporation of Massachusetts 'Applicatio n September 17, 1953, Serial No. 380,846

6 Claims. (Cl. 1 17-71) This invention relates to coating and in particular to the coating of metal substrates with molybdenum.

One object of the present invention is to produce adherent coatings of molybdenum on metal substrates.

Another object of the invention is to provide uniform adherent molybdenum coatings on the interior surfaces of hollow objects.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The'invention accordingly comprises the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of. the invention, reference should be had to the followingdetailed description taken in connection with the accompanying drawing which is a diagrammatic, schematic sectional view of one type of apparatus employed in practicing the invention.

Coating the surfaces of steel or other metal articles with molybdenum has recently become of great importance. A coating of molybdenum on metal articles often imparts to the surface of the article the desirable physical and chemical properties of molybdenum. Molybdenum-coated metals, such as steel,are often useful in connection with many types'of liquid or gaseous media 'where corrosion resistant surfaces are desirable. Such coated metal articles may also find'use where the surfaces are to be raised to very high temperatures.

The numerous applications to which molybdenumcoated articles may be adapted are dependent to 'a great extent on the adherence of the molybdenum to the metal substrate. Great difiiculty has been reported in the literature and experienced in many laboratories in obtaining uniform adherent molybdenum coatings on metal.

substrates, especially steel. It has been discovered that this poor adhesion is attributable to. (a): the fact that molybdenum often does not form a suitable diffusion bond with the metal substrate and (b) the deposit of a powdery material on the surface of the metal prior to coating with molybdenum.

The invention is, therefore, directed to a process for coating metal substrates and, in particular, the interior surfaces of hollow objects with a uniform adherent de-' posit of molybdenum. This may be accomplished by forming a suitable diffusion bond between the metal substrate and molybdenum and eliminating powdery material deposits on the metal surfaces which are about ,7 to be coated.

In the present invention, a uniform adherent coating of molybdenum may be obtained on a metal substrate,

particularly steel, by coating said metal substrate, prior to depositing molybdenum thereon, with a layer of cobalt which is capable of forming a diffusion bond with both the metal substrate and molybdenum. A coating of molybdenum, obtained by reducing molybdenum chlo 2,7833%- Patented Feb. 26, 1957 ride with hydrogen at a temperature-between about 800 C. and 1100 C., is then deposited on the thin I example is for illustration and is not to be construed in limiting the invention in any way. The reference numerals indicate the appropriate sections, of the drawing.

Example I A piece of steel pipe 2, thoroughly cleaned internally.

of all foreign substances, was coated with a layer 4 of cobalt to a depth of about 0.0004 inch, employing the chemical reduction method described by Brenner and Riddell in the Journal of Research, National Bureau of Standards, volume 39, 1947, page 388. Although the cobalt coating 4, produced by employing this method, will contain traces of combined phosphides, they willin no way affect the adherence of the subsequent molybdenum coating. The steel pipe 2 (internally coated with cobalt) was then placed in a vacuum system. Placed within the vpipe was a shield 6, which was positioned adjacent to and extending along the length of the in-, terior surface of the pipe. The shield 6 was moved to one extreme end of the pipe so as to expose the entire interior surface. The pipe 2 was then heated to 800 C. by means of an external heater 8, while dry hydrogen. was admitted at a pressure of 1 atmosphere so as to reduce any cobalt oxide present. After the entire pipe 2 had been heated and exposed to the dry hydrogen,

the pressure was reduced to 17 mm. of mercury abs. and the shield 6 was moved so as to cover the entire internal surface except for a small portion which was heated to 950 C. by means of the external heater 8. While continuing the passage of hydrogen, molybdenum chloride vapors obtained by heating a bed of molybdenum pentachloride at about C. were passed through the pipe 2. The partial pressure of the molybdenum pentachloride was maintained on the order of about 1 mm. Hg abs. during the coating operations.

Hydrogen reduction of the molybdenum chloride took place in the small heated exposed portion ltlso as to deposit thereon a coating of molybdenum. The shield 6 and the heater 8 were then moved in the same direction as the flow of hydrogen and molybdenum chloride vapors so as to expose and heat a new portionof the cobalt-coated surface to be coated with molybdenum.

' Under these conditions, an adherent coating of molyb:

denum was deposited on the cobalt along the entire interior surface of the pipe. The cobalt, during this latter coating operation, diffuses to a small extent into both siderably more effective coating is obtained with the co balt diffusion layer. Nickel is unsatisfactory as it tends to diffuse entirely into thesteel and the molybdenum so as to leave a weakening void where it was originally deposited. Cobalt does not diffuse to this extent and so does not leave any weakening voids.

It should also be noted that cobalt is more desirable than nickel in that cobalt-molybdenum alloys have greater ductility than nickel-molybdenum alloys and are thus better able to accommodate themselves to stresses introduced by cooling from the coating temperature. Moreover, microscopic examinations of the molybdenum structure deposited on either nickel or cobalt indicated that the structure deposited on cobalt is usually finer than that deposited on a nickel substrate and is, therefore, more able to withstand deformation.

The cobalt may be deposited on steel or other preferred metal substrates by a number of methods, such as electroplating, spraying, reduction of its chlorides, or by other chemical means. The thickness of the cobalt layer, however, should be carefully controlled so that only a thin film results. Cobalt films on the order of 0.0003 inch to 0.005 inch thick have been found to be satisfactory. Since the cobalt will diffuse to some extent into the steel. or other metal substrate, it is necessary to provide a cobalt film of sufficient depth so that no weakening voids will result. Films substantially less than 0.0003 inch thick may be unsatisfactory as voids may be formed due to the cobalt diffusion. Films substantially greater than 0.0005 inch may also be unsatisfactory, since there will be' formed an intermediate layer of cobalt of a substantial thickness. However, it should be mentioned that in some applications, an intermediate layer of cobalt may be desirable so that a thickness about 0.005 inch may in some cases be employed. The formation of too thick an intermediate layer aswell as too thin a layer may result in poor adhesion of the molybdenum.

The preferred metal substrates of th Present invention are those which are capable of forming a satisfactory diffusion bond with cobalt. Suitable substrates may consist of steel, nickel, or other metals whose melting point is above the temperature at which cobalt diffusion takes place.

The molybdenum is deposited over the cobalt layer and a diffusion bond betweenthe molybdenum and cobalt will be formed. The molybdenum coating is preferably deposited on the cobalt by reducing the chloride of molybdenum with hydrogen. Suitable molybdenum deposits may be obtained when the chloride is reduced at a temperature between about 800 C. and 1100 C., preferably above about 900 C., and at a total pressure between about and 25 mm. of mercury abs.

It should also be mentioned that although the cobalt will diffuse to some extent into the metal substrate and molybdenum during the coating operations, it may often be necessary to reheat the coated article so as to promote a further diffusion of the cobalt. This addiitonal treat ment may be desirable only where the coated article "requires an exceptionally adherent coating of molybdenum or where the surface is desired to be an alloy of cobalt and molybdenum.

When cobalt is deposited by electroplating methods or by the Brenner and Riddell chemical reduction method, it is desirable to pretreat the coating as described in the example. Cobalt deposited electrolytically usually con tains small quantities of cobalt oxide. The oxide is preferably removed by reducing it. in an atmosphere of a reducing gas, such as hydrogen. Although this isnot es sential to the invention, better results have been obtained when this reducing treatment was employed.

In experimcntscarried out in the absence of a shield, it was found that a powdery material was deposited on the surfaces beyond the heated reaction zone which were subsequently to be coated with molybdenum. This is very undesirable as it prevents theformation of a uniform adherent coating. This powdery material is believed to be composed of partially reacted molybdenum chloride vapors.

The shield is positioned adjacent to and extends along the length of the interior surfaces of the hollow object. Two methods of exposing and heating a new portion of the surface to be coated are possible. One preferred method is to employ a movable shield and external heater. Thus, during the coating operations, the shield and heater are moved at a predetermined rate and in the direction of the fiow of vapors. Another method is to employ a stationary shield and heater while moving the hollow object itself at a. predtermined rate and in a'direction opposite to the flow of the vapors. By moving the shield and heater or the hollow object in the above-mentioned directions, the reaction zone becomes positioned ahead of the shield so that any partially reacted material will fall on the shield and not on the surfaces to be coated.

The heating may be accomplished either by a gas burner, induction heating, or resistance heating. The heating element, however, should heat not only that small portion of the interior surface which is exposed but should also heat a small portion of the adjacent covered surface so that, as that surface is exposed, the proper temperature has already been attained and thus the reaction can proceed more smoothly and rapidly.

Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of producing an adherent molybdenum coating on a metal substrate capable of forming a diffusionbond with cobalt which comprises coating a thin layer of cobalt on the metal substrate and depositing on said cobalt layer a coating of molybdenum obtained by the hydrgen reduction of molybdenum chloride vapors.

2. The method of claim 1 wherein said metal substrate is steel.

3. The method of claim 1 wherein said molybdenum chloride vapors are reduced with hydrogen at a temperature between about 800 C. and 1100 C.

4. A method of producing an adherent molybdenum coating on the interior surfaces of a hollow metal object capable of forming a diffusion bond with'cobalt which comprisescoating said interior surface with a thin layer of cobalt and depositing on said cobalt layer a coating of molybdenum obtained by the hydrogen reduction of molybdenum chloride vapors.

5. The method of claim 4 wherein said cobalt coating has a thickness on the order of between 0.0003 inch and 0.005ir1 ch.

6. The method of claim 4 wherein said molybdenum chloride vapors are reduced with hydrogen at a temperature between about 800 C. and 1100 C.

References Cited in the file of this patent UNITED STATES PATENTS 1,977,639 Langdon Oct. 23, 1934 2,296,838 Domm Sept. 29, 1942 2,373,639 Turner Apr. 10, 1945 2,426,016 Gustin Aug. 19, 1947 2,516,058 Lander July 18, 1950 2,671,034 Steinfeld Mar. 2, 1954 OTHER REFERENCES Childs, et al.: Transactions of American Society for Metals, vol. 43 (ll),pp. -421. 

1. A METHOD OF PRODUCING AN ADHERENT MOLYBDENUM COATING ON A METAL SUBSSTRATE CAPABLE OF FORMING A DIFFUSION BOND WITH COBALT WHICH COMPRISES COATING A THIN LAYER OF COBALT ON THE METAL SUBSTRATE AND DEPOSITING ON SAID COBALT LAYER A COATING OF MOLYBDENUM OBTAINED BY THE HYDRGEN REDUCTION OF MOLYBDENUM CHLORIDE VAPORS. 